The present invention relates to the field of measuring devices, specifically to non-contact devices for measuring the dimensions of a cylindrical object, such as a pipe. Pipe will be used as an exemplary work piece throughout this disclosure. During the process of manufacturing pipe, it is desirable to measure the dimensions of the pipe to ensure compliance with predetermined tolerances for quality control. Of particular concern are the dimensions of the pipe at its end that will form a juncture with a second piece of pipe. This end is referred to as the bell, and there are several dimensions on the bell that are critical to forming a proper juncture with another pipe. The measurements of the bell have traditionally been performed with simple "go/no-go" gauges, or at times with calipers.
Go/no-go gauges are mechanical articles of a fixed size which, when inserted (or attempted to be inserted) into a pipe will fit properly or not, thus indicating whether the pipe under test is acceptable. These gauges check the minimum or maximum dimensions at certain critical points in the bell, but they do not provide actual bell dimensions. Calipers provide actual dimensions, but there is simply not enough time in a modern manufacturing process to measure the pipe with calipers or any other standard measuring device. Moreover, most pipes are not truly circular. Therefore, in order to determine the shape of the pipe, one would have to make many measurements with calipers around its perimeter, increasing the time even more. It would be desirable to determine the shape of a pipe in order to identify precisely any sections that need to be ground in order to bring the pipe into conformity with manufacturing tolerances. In addition, if the shape of each pipe manufactured in the process were recorded, statistical analysis of the manufacturing process could be performed. Such statistical analysis is not feasible with a manual measurement system based on calipers or go/no-go gauges. As with any manual system, human error is pervasive. An automated system is needed.
Some automated contact systems are available. Such systems typically use some form of linear variable differential transformer or linear potentiometer, both of which require a sensor to touch the surface of the unit under test. However, the interior surface of a pipe may be abrasive, and in a volume manufacturing process the sensor would wear out in an unacceptably short time. Using such contact sensors therefore would increase maintenance costs and process down time, while decreasing the reliability, and over time, the precision of the measurement system. Moreover, the range of most contact sensors is rather limited and is insufficient to measure large diameter pipe. As discussed previously, there are several points of interest on the pipe bell. To measure multiple points with a contact system, the sensor would need to be removed from the surface, repositioned to the new location, and then placed in contact with the surface again. This procedure would prove too slow for a volume manufacturing process.
Moreover, those contact systems of which the inventor is aware rely on placing a device in the center of the pipe in order to measure it. However, as noted above, few pipes are truly circular in cross section. Thus, any measurement system which relies on placing the measurement device at the center of the pipe is based on a flawed premise.
Thus, there exists a need for an automated, non-contact measurement system for measuring the inside dimensions of pipe; that will provide and record precise, actual dimensions of the pipe being measured; that will provide the cross-sectional shape of the pipe for statistical analysis and grinding purposes; that will perform the measurement process in a fast manner suitable for a modem, volume manufacturing process; and that will do so without requiring the measurement device to be placed at the center of the pipe.